Both electromagnets and permanent magnets have been used to manipulate beams of charged particles. In traveling wave tubes, for example, magnets have been arranged around the channel through which the beam travels to focus the stream of electrons; that is, to reduce the tendency of the electrons to repel each other and spread out. Various configurations of permanent magnets (and pole pieces) have been tried in an attempt to increase the focusing effect while minimizing the weight and volume of the resulting device. In conventional traveling wave tubes, permanent magnets are often arranged in a sequence of alternating magnetization, either parallel to, or anti-parallel to, the direction of the electron flow. These axially magnetized, permanent magnets are usually annular or toroidal in shape and their axes are aligned with the path of the electron beam. The patent to Clarke, U.S. Pat. No. 4,731,598, issued Mar. 15, 1988, illustrates typical prior art, periodic permanent magnet (PPM) structures.
An axially magnetized toroidal ring is typically made by subjecting a ring of magnetic material to an intense magnetic field using a very large electromagnetic source. To provide an intense magnetic field (e.g., 13 kO.sub.e) for this purpose the electromagnetic source is, of necessity, large (several hundred pounds), cumbersome, and requires high input power.
There are instances and/or applications where radially magnetized toroidal rings are desirable. Heretofore, the making of radially magnetized toroids was difficult and time consuming. Typically, a plurality of toroid sections were magnetized piece-by-piece and the magnetized sections then assembled to form a radially magnetized toroidal ring. But, unfortunately, this laborious technique still provides only an approximation to a true radial field. In a true radial magnetic field the direction of magnetization changes continuously around the toroidal circle. However, with a sectioned toroid, significant field discontinuities occur from section to section.
There are also some limited situations which call for a toroidal ring with a field direction at some selected angle with respect to the toroid axis. For example, ring-shaped bucking corner magnets mounted on the ends of a cylindrical primary magnet usually require a field direction 45.degree. with respect to the axis of the primary magnet. However, to magnetize a toroidal ring at some arbitrary angle with respect to the toroid axis is done only with great difficulty and only in the described section-by-section manner. Besides fabrication difficulties, the field discontinuities encountered have proved troublesome.